Heretofore, there have been known thermionic converters such as those shown in U.S. Pat. Nos. 3,519,854, 3,328,611, 4,303,845, 4,323,808, 5,459,367, 5,780,954 and 5,942,834 (all to the inventor of the present invention and all hereby incorporated by reference), which disclose various apparatus and methods for the direct conversion of thermal energy to electrical energy. In U.S. Pat. No. 3,519,854, there is described a converter using Hall effect techniques as the output current collection means. The '854 patent teaches use of a stream of electrons boiled off of an emissive cathode surface as the source of electrons. The electrons are accelerated toward an anode positioned beyond the Hall effect transducer. The anode of the '854 patent is a simple metallic plate, which has a heavily static charged member circling the plate and insulated from it.
U.S. Pat. No. 3,328,611 discloses a spherically configured thermionic converter, wherein a spherical emissive cathode is supplied with heat, thereby emitting electrons to a concentrically positioned, spherical anode under the influence of a control member, the spherical anode having a high positive potential thereon and insulated from the control member. As with the '854 patent, the anode of the '611 patent is simply a metallic surface.
U.S. Pat. No. 4,303,845 discloses a thermionic converter wherein the electron stream from the cathode passes through an air core induction coil located within a transverse magnetic field, thereby generating an EMF in the induction coil by interaction of the electron stream with the transverse magnetic field. The anode of the '845 patent also comprises a metallic plate which has a heavily static charged member circling the plate and insulated from it.
U.S. Pat. No. 4,323,808 discloses a laser-excited thermionic converter that is very similar to the thermionic converter disclosed in the '845 patent. The main difference is that the '808 patent discloses using a laser which is applied to a grid on which electrons are collected at the same time the potential to the grid is removed, thereby creating electron boluses that are accelerated toward the anode through an air core induction coil located within a transverse magnetic field. The anode of the '808 patent is the same as that disclosed in the '845 patent, i.e., simply a metallic plate which has a heavily static charged member circling the plate and insulated from it.
U.S. Pat. No. 5,459,367 advantageously uses an improved collector element with an anode having copper wool fibers and copper sulfate gel instead of a metallic plate. Additionally, the collector element has a highly charged (i.e., static electricity) member surrounding the anode and insulated from it.
U.S. Pat. Nos. 5,780,954 and 5,942,834 are directed to the provision of a cathode that is constructed as a wire grid, with the cathode being of a non-planar shape to increase its emissive surface area. These patents also disclose the technique of using a laser to hit the stream of electrons before they reach the anode, as a measure of providing quantum interference such that the electronics may be more readily captured by the anode.
Another prior design has an anode and cathode which are relatively close together such as two microns apart within a vacuum chamber. Such a prior design uses no attractive force to attract electrons emitted from the cathode to the anode other than induction of cesium into the chamber housing the anode and cathode. The cesium coats the anode with a positive charge to keep the electrons flowing. With the cathode and anode so close together, it is difficult to maintain the temperatures of the cathode and anode at substantially different temperatures. For example, one would normally have the cathode at 1800 degrees Kelvin and the anode at 800 degrees Kelvin. A heat source is provided to heat the cathode and a coolant circulation system is provided at the anode in order to maintain it at the desired temperature. Even though the chamber is maintained at a vacuum (other than the cesium source), heat from the cathode goes to the anode and it takes a significant amount of energy to maintain the high temperature differential between the closely spaced cathode and anode. This in turn lowers the efficiency of the system substantially.